This invention relates to a regulator for regulating charging of a battery from the alternating current output terminals of a permanent magnet alternator such as one that is driven by the engine of a vehicle or boat.
As is well known, the alternating output voltage and output current from a permanent magnet alternator are directly related to the rotational speed of the alternator and, hence, to the speed at which the engine is running. When a permanent magnet alternator is used to charge a storage battery, a regulator must be used to limit the current delivered to the battery to avoid overcharging. Since the field flux of a permanent magnet alternator is constant and not easily changed, the output circuit that is connected to the alternator stator winding must be opened or short circuited to regulate the voltage rather than by varying field excitation current as is possible in wound rotor alternators.
Short circuiting of the output of the alternator to achieve regulation of the voltage that is applied to the battery for charging it has several disadvantages. One is that the alternating windings tend to overheat under short circuit conditions as a result of the heavy current flow. Another is that the alternator ac output signal is lost which in many instances is used for other purposes such as providing pulses to a tachometer circuit. There are numerous prior patents that disclose regulators which regulate voltage by shunting or short circuiting the alternator.
A method that is preferred over shunting the alternator output is to charge the battery from the alternator output through a diode or rectifier bridge wherein thyristor switches such as silicon controlled rectifiers (SCRs) are used in the legs of the bridge. Generally, if a half wave rectifier is used, an SCR will be connected in a single series circuit with the battery and if the circuit is for full wave rectification, there will be an SCR in alternate legs of the full wave rectifier bridge. Means are typically provided for sensing battery voltage. When a voltage below full charge level is sensed, the SCRs are gated on and charging current is delivered to the battery until fully charged voltage is sensed in which case the gating signal is removed from the SCRs and no further charging current flows. These regulators depend on the terminal voltage of the battery dropping below some fixed level before any charging begins. They are incapable of providing a low charging rate when battery voltage is just slightly below fully charged voltage and charging at a high rate when battery terminal voltage is substantially below that which corresponds to fully charged condition.
U.S. Pat. Nos. 3,857,082 and 4,146,831 typify the use of full-wave bridge rectifiers for charging a battery from the output terminals of a permanent magnet alternator. In both cases, the rectifying diode in two of the legs of the bridge consists of an SCR that provides a gate terminal. Transistor circuits are used for sensing battery voltage and responding to battery voltage dropping down to a particular level by gating the SCRs on to charge the battery with full-wave dc. When fully charged battery voltage is sensed, the SCRs are turned off and the flow of charging current to the battery is completely interrupted. This is amply demonstrated in FIG. 6 of U.S. Pat. No. 3,857,082 and in FIG. 1 of U.S. Pat. No. 4,146,831.